An adaptive array radio base station employing an array antenna is recently put into practice as a radio base station for a mobile communication system such as a portable telephone. The operation principle of such an adaptive array radio base station is described in the following literature, for example:
B. Widrow, et al.: “Adaptive Antenna Systems”, Proc. IEEE, vol.55, No.12, pp.2143-2159 (December 1967).
S. P. Applebaum: “Adaptive Arrays”, IEEE Trans. Antennas & Propag, vol.AP-24, No.5, pp.585-598 (September 1976).
O. L. Frost, III: “Adaptive Least Squares Optimization Subject to Linear Equality Constraints”, SEL-70-055, Technical Report, No.6796-2, Information System Lab., Stanford Univ. (August 1970).
B. Widrow and S. D. Stearns: “Adaptive Signal Processing”, Prentice-Hall, Englewood Cliffs (1985).
R. A. Monzingo and T. W. Miller: “Introduction to Adaptive Arrays”, John Wiley & Sons, New York (1980).
J. E. Hudson: “Adaptive Array Principles”, Peter Peregrinus Ltd., London (1981).
R. T. Compton, Jr.: “Adaptive Antennas—Concepts and Performance”, Prentice-Hall, Englewood Cliffs (1988).
E. Nicolau and D. Zaharia: “Adaptive Arrays”, Elsevier, Amsterdam (1989).
FIG. 68 is a model diagram conceptually showing the operation principle of such an adaptive array radio base station. Referring to FIG. 68, an adaptive array radio base station 1 comprises an array antenna 2 formed by n antennas #1, #2, #3, . . . , #n, and a first diagonal line area 3 shows a range in which radio waves from the array antenna 2 can be received. A second diagonal line area 7 shows a range in which radio waves from adjacent another radio base station 6 can be received.
In the area 3, the adaptive array radio base station 1 transmits/receives a radio signal to/from a portable telephone 4 forming a terminal of a user A (arrow 5). In the area 7, the radio base station 6 transmits/receives a radio signal to/from a portable telephone 8 forming a terminal of another use B (arrow 9).
When the radio signal for the portable telephone 4 of the user A happens to be equal in frequency to the radio signal for the portable telephone 8 of the user B, it follows that the radio signal from the portable telephone 8 of the user B serves as an unnecessary interference signal in the area 3 depending on the position of the user B, to disadvantageously mix into the radio signal transmitted between the portable telephone 4 of the user A and the adaptive array radio base station 1.
In this case, it follows that the adaptive array radio base station 1 receiving the mixed radio signals from both users A and B in the aforementioned manner outputs the signals from the users A and B in a mixed state unless some necessary processing is performed, to disadvantageously hinder communication with the regular user A.
In order to eliminate the signal from the user B from the output signal, the adaptive array radio base station 1 performs the following processing. FIG. 69 is a schematic block diagram showing the structure of the adaptive array radio base station 1.
Assuming that A(t) represents the signal from the user A and B(t) represents the signal from the user B, a signal x1(t) received in the first antenna #1 forming the array antenna 2 shown in FIG. 68 is expressed as follows:x1(t)=a1×A(t)+b1×B(t)where a1 and b1 represent factors changing in real time, as described later.
A signal x2(t) received in the second antenna #2 is expressed as follows:
x2(t)=a2×A(t)+b2×B(t)
where a2 and b2 also represent factors changing in real time.
A signal x3(t) received in the third antenna #3 is expressed as follows:
x3(t)=a3×A(t)+b3×B(t)
where a3 and b3 also represent factors changing in real time.
Similarly, a signal xn(t) received in the n-th antenna #n is expressed as follows:xn(t)=an×A(t)+bn×B(t)where an and bn also represent factors changing in real time.
The above factors a1, a2, a3, . . . , an show that the antennas #1, #2, #3, . . . , #n forming the array antenna 2 are different in receiving strength from each other with respect to the radio signal from the user A since the relative positions of the antennas #1, #2, #3, . . . , #n are different from each other (the antennas #1, #2, #3, . . . , #n are arranged at intervals about five times the wavelength of the radio signal, i.e., about 1 m, from eath other).
The above factors b1, b2, b3, . . . , bn also show that the antennas #1, #2, #3, . . . , #n are different in receiving strength from each other with respect to the radio signal from the user B. The users A and B are moving and hence these factors a1, a2, a3, . . . , an and b1, b2, b3, . . . , bn change in real time.
The signals x1(t), x2(t), x3(t), . . . , xn(t) received in the respective antennas #1, #2, #3, . . . , #n are input in a receiving unit 1R forming the adaptive array radio base station 1 through corresponding switches 10-1, 10-2, 10-3, . . . , 10-n respectively so that the received signals are supplied to a weight vector control unit 11 and to one inputs of corresponding multipliers 12-1, 12-2, 12-3, . . . , 12-n respectively.
Weights w1, w2, w3, . . . , wn for the signals x1(t), x2(t), x3(t), . . . , xn(t) received in the antennas #1, #2, #3, . . . , #n are applied to other inputs of these multipliers 12-1, 12-2, 12-3, . . . , 12-n respectively. The weight vector control unit 11 calculates these weights w1, w2, w3, . . . , wn in real time, as described later.
Therefore, the signal x1(t) received in the antenna #1 is converted to w1×(a1A(t)+b1B(t)) through the multiplier 12-1, the signal x2(t) received in the antenna #2 is converted to w2×(a2A(t)+b2B(t)) through the multiplier 12-2, the signal x3(t) received in the antenna #3 is converted to w3×(a3A(t)+b3B(t)) through the multiplier 12-3, and the signal xn(t) received in the antenna #n is converted to wn×(anA(t)+bnB(t)) through the multiplier 12-n.
An adder 13 adds the outputs of these multipliers 12-1, 12-2, 12-3, . . . , 12-n, and outputs the following signal:                w1(a1A(t)+b1B(t))+w2(a2A(t)+b2B(t))+w3(a3A(t)+b3B(t))+ . . . +wn(anA(t)+bnB(t))        
This expression is classified into terms related to the signals A(t) and B(t) respectively as follows:                (w1a1+w2a2+w3a3+ . . . +wnan)A(t)+(w1b1+w2b2+w3b3+ . . . +wnbn)B(t)        
As described later, the adaptive array radio base station 1 identifies the users A and B and calculates the aforementioned weights w1, w2, w3, . . . , wn to be capable of extracting only the signal from the desired user. Referring to FIG. 69, for example, the weight vector control unit 11 regards the factors a1, a2, a3, . . . , an and b1, b2, b3, . . . , bn as constants and calculates the weights w1, w2, w3, . . . , wn so that the factors of the signals A(t) and B(t) are 1 and 0 as a whole respectively, in order to extract only the signal A(t) from the intended user A for communication.
In other words, the weight vector control unit 11 solves the following simultaneous linear equations, thereby calculating the weights w1, w2, w3, . . . , wn so that the factors of the signals A(t) and B(t) are 1 and 0 respectively:                w1a1+w2a2+w3a3+ . . . +wnan=1        w1b1+w2b2+w3b3+ . . . +wnbn=0        
The method of solving the above simultaneous linear equations, not described in this specification, is known as described in the aforementioned literature and already put into practice in an actual adaptive array radio base station.
When setting the weights w1, w2, w3, . . . , wn in the aforementioned manner, the adder 13 outputs the following signal:                output signal=1×A(t)+0×B(t)=A(t)        
The aforementioned users A and B are identified as follows: FIG. 70 is a schematic diagram showing the frame structure of a radio signal for a portable telephone set. The radio signal for the portable telephone set is roughly formed by a preamble consisting of a signal series known to the radio base station and data (sound etc.) consisting of a signal series unknown to the radio base station.
The signal series of the preamble includes a signal series of information for recognizing whether or not the user is a desired user for making communication with the radio base station. The weight vector control unit 11 (FIG. 69) of the adaptive array radio base station 1 compares a training signal corresponding to the user A fetched from a memory 14 with the received signal series and performs weight vector control (decision of weights) for extracting a signal apparently including the signal series corresponding to the user A. The adaptive array radio base station 1 outputs the signal from the user A extracted in the aforementioned manner as an output signal SRX(t).
Referring again to FIG. 69, an external input signal STX(t) is input in a transmission unit 1T forming the adaptive array radio base station 1 and supplied to one inputs of multipliers 15-1, 15-2, 15-3, . . . , 15-n. The weights w1, w2, w3, . . . , wn previously calculated by the weight vector control unit 11 on the basis of the received signal are copied and applied to other inputs of these multipliers 15-1, 15-2, 15-3, . . . , 15-n respectively.
The input signal STX(t) weighted by these multipliers 15-1, 15-2, 15-3, . . . , 15-n is sent to the corresponding antennas #1, #2, #3, . . . , #n through corresponding switches 10-1, 10-2, 10-3, . . . , 10-n respectively, and transmitted into the area 3 shown in FIG. 68.
The signal transmitted through the same array antenna 2 as that in receiving is weighted for the target user A similarly to the received signal, and hence the portable telephone set 4 of the user A receives the transmitted radio signal as if the signal has directivity to the user A. FIG. 71 images such transfer of a radio signal between the user A and the adaptive array radio base station 1. Imaged is such a state that the adaptive array radio base station 1 transmits the radio signal with directivity toward the target portable telephone set 4 of the user A as shown in a virtual area 3a in FIG. 71 in contrast with the area 3 of FIG. 68 showing the range actually receiving radio waves.
In order to implement such transmission/receiving of the radio signal with directivity between the desired user and the adaptive array radio base station 1, the adaptive array radio base station 1 must strictly calculate the weights w1, w2, w3, . . . , wn for equivalently weighting received and transmitted signals in the receiving unit 1R and the transmission unit 1T. Even if the adaptive array radio base station 1 completely controls weighting, however, the transmission characteristics of the transmitted signal may change with respect to the received signal such that the transmitted signal cannot be transmitted to the target user.
In the adaptive array radio base station 1 shown in FIG. 69, for example, the distance between the switches 10-1, 10-2, 10-3, . . . , 10-n and the corresponding multipliers 12-1, 12-2, 12-3, . . . , 12-n of the receiving unit 1R and the distance between the switches 10-1, 10-2, 10-3, . . . , 10-n and the corresponding multipliers 15-1, 15-2, 15-3, . . . , 15-n of the transmission unit 1T are generally not completely identical to each other. If the distances are different from each other, a difference in phase rotation quantity, a difference in amplitude fluctuation quantity, or the like is disadvantageously caused between the received signal and the transmitted signal received in and transmitted from each antenna, and the radio signal cannot be transferred between the target user and the adaptive array radio base station 1 with excellent directivity.
In general, paths between the switches 10-1, 10-2, 10-3, . . . , 10-n and the corresponding multipliers 12-1, 12-2, 12-3, . . . , 12-n of the receiving unit 1R include necessary receiving circuits (not shown) respectively, and paths between these switches 10-1, 10-2, 10-3, . . . , 10-n and the corresponding multipliers 15-1, 15-2, 15-3, . . . , 15-n of the transmission unit 1T include necessary transmission circuits (not shown) respectively. Therefore, it follows that a difference in phase rotation quantity, difference in amplitude fluctuation quantity, or the like is caused between the received signal and the transmitted signal received in and transmitted from each antenna depending on the characteristics of amplifiers, filters etc. forming the receiving and transmission circuits.
In the adaptive array radio base station 1, therefore, the transmission characteristics of the receiving circuit such as the phase rotation quantity and the amplitude fluctuation quantity and the transmission characteristics of the transmission circuit such as the phase rotation quantity and the amplitude fluctuation quantity must be measured as to each antenna forming the array antenna 2, for compensating for the differences. In general, a measuring circuit for measuring the transmission characteristics is separately provided on the adaptive array radio base station, to disadvantageously enlarge and complicate the circuit structure of the adaptive array radio base station while increasing the cost.
An object of the present invention is to provide a radio apparatus capable of estimating and compensating for a difference between transmission characteristics of a receiving circuit and a transmission circuit in a simple structure at a low cost without providing a specific measuring circuit and a calibration method therefor.